Electronic devices, such as mobile phones and digital cameras, equipped with a microphone have been widely used. Generally, the microphone of these electronic devices is a compact capacitor microphone. Usually, the microphone is accommodated in a casing of the electronic device as a microphone unit in which a sound transducer (sound-collecting part) having a diaphragm and a back plate is accommodated in a housing of the unit. The sound from the outside is guided to the sound transducer of the microphone through both a sound-collecting opening provided to the casing and a sound-collecting opening provided to the housing. When a foreign matter, such as dust, enters into the microphone through these sound-collecting openings, the vibration of the foreign matter occurs in the vicinity of the sound transducer, generating noise. In addition, the entry of a foreign matter into the sound transducer leads to a failure of the microphone. Therefore, a sound-transmitting membrane that allows sound to transmit through the sound-transmitting membrane while preventing the entry of the foreign matter generally is disposed at the sound-collecting opening of at least one member selected from the casing and the housing. Disposing the sound-transmitting membrane also suppresses the noise generated by the blow-in of wind or breath.
Conventionally, a porous sheet having air permeability is used as the sound-transmitting membrane. JP 2008-199225 A discloses that a woven fabric or nonwoven fabric produced from fibers made of a resin, such as nylon and polyethylene, can be used as a sound-transmitting membrane. JP 2007-81881 A discloses that a porous polytetrafluoroethylene (PTFE) membrane can be used as a sound-transmitting membrane. The latter sound-transmitting membrane is expected to suppress the entry of water as well as dust, as foreign matters, judging from the properties of the porous PTFE membrane.